Numerous operations are performed on oil and gas wells which require large volumes of nitrogen gas or other cryogenic fluids. These operations may be performed on both onshore and offshore wells. Such operations include foam fracturing operations, acidizing services, jetting down the tubing or down the tubing-casing annulus, nitrogen cushions for drill stem testing, pressure testing, insulation of the tubing-casing annulus to prevent such problems as paraffin precipitation, jetting with proppant for perforating and cutting operations, reduction of density of well workover fluids, displacement of well fluid from tubing during gun perforation operations to prevent excess hydrostatic pressure in the hole from pushing perforation debris into the formation, placing corrosion inhibitors by misting the inhibitor with nitrogen, extinguishing well fires, and other operations. Other operations that require cryogenic fluids at a high temperatures up to 400.degree. F. include pipeline purging and drying operations, and refinery operations such as, recharging catalysts.
Nitrogen is typically stored in its liquid state because of the volume used however, liquid nitrogen will damage most carbon steel pipes used in oil and gas wells. Thus, various heating systems have been developed to raise the nitrogen to an ambient temperature. Typically, 185 BTUs per pound of nitrogen are required to heat the nitrogen to an ambient temperature of 70.degree. F.
One particular such operation is the fracturing of a subsurface formation of the well by pumping a fluid under very high pressure into the formation. The fracturing fluid which is pumped into the well often comprises a foamed gel which is produced by the use of nitrogen gas. The nitrogen for the foam fracturing operation is generally stored in a fluid form at temperatures of approximately .sup.- 320.degree. F.
At pressures encountered in these foam fracturing operations, the nitrogen changes state from a liquid to a gas at approximately .sup.- 200.degree. F. It is, therefore, desirable to heat up the nitrogen gas so that the foam fracturing fluid being pumped down the well will be at an essentially ambient temperature. This is because of the numerous adverse affects upon mechanical equipment at very low temperature.
With regard to oil well operations, the nitrogen heating equipment generally includes open flame heaters. For example, with currently available technology, the only way to elevate the temperature of nitrogen to 400.degree. F. is by using devices such as burners or direct fired unit-open flame units. For safety and environmental reasons, open flame heaters are generally not desirable or acceptable on offshore and onshore operations. Open flame heaters are banned from certain operations because they generate sparks and flames that could ignite surrounding vapors and typically the surface temperature on these units is above 250.degree. F. which also creates safety hazards. Therefore, there is a need for a heater for the nitrogen which does not have an open flame and the heater does not generate surface temperatures above 250.degree. F.
Such flameless nitrogen heaters have previously been provided by utilizing the heat generated by an internal combustion engine and mechanical components driven thereby to heat a coolant fluid which then transferred its heat to the nitrogen through a series of heat exchangers. Multiple heat exchangers ultimately add to the weight, space and cost of the unit. In addition, these heat exchangers typically do not arrest all of the flames and sparks emitted by the exhaust from the engine.
Numerous problems are encountered with other prior art devices mainly because of the use of air as a heat transfer medium. Air is a notoriously poor heat transfer medium as compared to a liquid. Additionally, due to the large bulky nature of the plenum chamber required for the use of air as a heat transfer medium, the air systems are typically very bulky and heavy.
Therefore, there is a need for a flameless nitrogen unit that is compact in size, efficient in the heat transfer process, and economical. There is also a need for a system that heats fluids to a desired temperature while quenching sparks and maintaining surface temperatures and exhaust temperatures at acceptable levels.